1. Field of the Invention
The invention relates to an electronic unit in which an electronic body is housed in a casing.
2. Description of Related Art
In a conventional vehicular lamp, such as a headlamp of a vehicle, for example, use of a discharge bulb as a light source has been proposed. Therefore, an electronic unit in which a circuit for controlling the lighting of this discharge lamp is house in a casing is formed, and this electronic unit is arranged in a lamp housing. Japanese Patent No. 4087396 proposes technology in which an electronic unit having such a structure is formed detachable with respect to the lamp housing. In recent years, many lamps using semiconductor light-emitting devices, such as an LED (Light Emitting Diode) for example, for the light source have been proposed. Therefore, an electronic unit has been proposed in which a LED drive circuit for making this LED emit light is formed housed, together with an electronic control unit (ECU) for controlling this light emission, in a single casing, and arranged in a lamp housing of the lamp.
With such an electronic unit, parts having high heat generation properties such as transistors and coils (hereinafter referred to as “heat generating parts”) are used in a light emitting circuit for making the LED emit light, so the temperature of the electronic unit tends to rise from the heat generated by these heat generating parts. When the temperature rises in this way, it may adversely affect normal lighting control of the electronic unit, and various electronic components that form the electronic unit may become heat damaged. In particular, with a lamp having a structure in which the electronic unit is mounted to an external portion of the lamp housing, the casing must be formed as a sealed chamber in order to prevent outside dust from getting into the electronic unit. Therefore, heat generated inside the casing is not easily dissipated to the outside, so the electronic unit does not dissipate heat easily. In order to improve the heat dissipation effect, the casing is preferably made of material having high thermal conductivity, such as metal, for example, but this kind of material increases the weight as well as makes the manufacturing process of the casing complex, and thus increases the cost.